The present invention relates to a device for transmitting high-frequency signals and to a method for the manufacture thereof.
In communication systems it is increasingly required to transmit and process electrical signals at a very high rate. The electrical signals are formed here as control signals and/or communication signals, in particular. Particularly in communication signals which are transmitted in asynchronous transfer mode (ATM), transmission rates of up to 830 MBit/s can arise. But problems arise in the processing of such high transmission speeds which are unknown in the transmission of data at low speeds.
In general, the transmission of signals entails the problem that electrical signals which are transmitted on lines residing in close proximity to one another, usually on copper cables, couple electromagnetically. At lower frequencies, electromagnetic coupling of the communication signals running on the copper cables does not occur or, is insignificant. However, given ever higher transmission frequencies, such a coupling can no longer be avoided. Couplings result in electromagnetic interference, for example. A suitable shielding could help in this regard, but to take this course of action on every cable in the communication system would be uneconomical.
Therefore, in order to keep potential couplings to a minimum, a complementary transmission of the communication signals via a data line is desired. This line is formed by a cable pair.
This means that one pulse train is transmitted over one of the lines of the cable pair, while the complementary pulse train is transmitted over the other line of the cable pair. Here, a possible crosstalk of signals that are transmitted over neighboring lines causes disturbances on both lines in the same manner. These are then averaged out again at the receive side by forming difference signals. This type of data transmission thus has a relatively high protection against potentially arising interference.
The assemblies or printed circuit boards of longstanding use in communication systems comprise connecting structures in the prior art, where copper tracks run parallel to the edge of the respective printed board (edge-parallel). The relevant electrical signals are then emitted via these copper tracks.
The printed boards used are produced from a material comprising a glass mat. The glass mats themselves consist of individual optical fiber bundles which are soaked in resin and fashioned into a web structure. Copper tracks run at the surface of a glass mat that is formed in this way, via which the electrical signals are conducted in practical operation. The glass mat thus serves as carrier of the copper tracks. In order to obtain a compact construction, a majority of these glass mats with pertaining copper lines are glued together. This means that the majority of these copper tracks runs within the thus formed "layer blocks". The latter then form the actual printed board. Terminals occur at the surface side by means of through-plating.
At very high frequencies, such as are used in the transmission of ATM communication signals, a complementary transmission of the communication signals over a data line is controlled, as already discussed. The density of the copper traces inside the printed circuit board is extremely high. In order to construct the complementary transmission of the communication signals over a data line in a space-efficient manner, a data line is respectively formed from a copper trace pair, as mentioned above. A thoroughly desirable electromagnetic coupling then occurs, since the field lines of the two traces encompass each other.
But a problem arises here that the structure of the interconnects attains the dimension of the web structure. The possibility thus arises that one trace of the pair runs in the vicinity of the optical fiber, while the other trace of the respective pair runs in the resin itself. The two materials have different dielectricity constants, however. Since an edge-parallel leading of the copper traces is provided in this prior art, this means that the copper tracks can run either in the vicinity of an optical fiber or in the resin itself.
But this results in different signal transit times in the transmission process, both in the two copper traces of a pair and between the data lines as well. For a better understanding, the corresponding relations are illustrated in FIG. 3. For example, the differential amplifier, which is arranged at the end of the two copper traces of a pair and which forms the difference signal, receives the edges of the pulses which are transmitting in a complementary fashion at different instants, and on its own part it generates a potentially flat edge or, edge shift therefrom, which can no longer be used to trigger other mechanisms.
One solution to this problem is to use homogenous materials having one dielectricity constant, however, these are expensive.
European Patent Application No. 0 309 982 teaches a substrate material and a method for its production in that the problem of the transit time difference is solved in that the substrate material that is produced with the method consists of several layers and that the fiber bundles of the individual layers are arranged at a specific angle to one another. This produces a quasi-isotropy. In any case, this reference does not contain data lines which conduct electrical signals or the arranging thereof in relation to the fiber bundles. This is unnecessary, since it is not possible for transit time differences between the individual data lines to occur, due to the quasi-isotropy.